Flotation is a common method for separating mixtures. It is commonly employed in the mining field to separate solids. The minerals such as ores or coal are pulverized and then subjected to separation methods such as froth flotation. The fine particles are mixed with water to form a slurry and air is bubbled through the slurry to produce a froth which typically contains the desired mineral while the remainder of the slurry contains the unwanted materials. Chemical additives such as surfactants may be added to improve the separation. The froth may then be dewatered by filtration or gravity separation. Flotation techniques are also widely employed in paper production and water treatment. In industrial waste water treatment, fats and oils are separated from water using treatment units such as dissolved air flotation (DAF) units.
Haze formation in lubricant oil base stocks is typically associated with molecules having some paraffinic characteristics, e.g., waxy molecules and molecules having long paraffinic chains. Lubricant oil base stocks are conventionally prepared by various combinations of hydrotreating, hydrocracking, solvent extraction, solvent deasphalting, solvent dewaxing, catalytic dewaxing, and hydrofinishing. Waxy molecules in the lubricant oil feed stock may be at least partially removed by solvent dewaxing or catalytic dewaxing. Solvent dewaxing typically involves mixing with solvents, usually at atmospheric pressure, separating wax that precipitates, and recycling recovered solvent. The solvent is usually chilled prior to addition to the dewaxing solvent, usually in a cooling tower. Representative solvents include aliphatic ketones, low molecular weight hydrocarbons and mixtures with aromatic solvents such as benzene, toluene or xylene.
Catalytic dewaxing involves contacting the feed to be dewaxed with a dewaxing catalyst under dewaxing conditions. Dewaxing catalysts usually function primarily by cracking or primarily by isomerization. Cracking dewaxing catalysts remove waxes by cracking them to molecules having lower molecular weights. Some yield loss occurs while using cracking dewaxing catalysts as such catalysts normally involve some cracking to molecules outside the lubricating oil range. ZSM-5 is an example of a dewaxing catalyst that normally functions primarily by cracking. Catalysts which function primarily by isomerization, e.g., ZSM-48, isomerize the paraffinic waxy molecules to more highly branched molecules. These isomerized molecules generally have more favorable properties with regard to viscosity and pour points.
Regardless of how dewaxing is accomplished, it is typical to follow dewaxing with a further step to remove small amounts of color bodies or haze forming bodies that remain after or are formed during dewaxing. Haze forming precursors result in haze typically upon standing. Haze is more of a problem at lower temperatures. These haze forming precursors generally have waxy character but are not necessarily simple long-chain molecules associated with wax. Such precursors may include cyclic and heterocyclic moieties to which are attached side chains having waxy paraffin character. Haze precursors can be removed by hydrofinishing. Hydrofinishing is a catalytic process and may be considered as a form of mild hydrotreating. Hydrofinishing may involve the same catalysts that are used in hydrotreating although at generally lower temperatures. Hydrofinishing may also be accomplished using the M41S family of mesoporous catalysts such as MCM-41, MCM-48 and MCM-50. U.S. Pat. No. 6,579,441 describes a process for dehazing a base oil using solid adsorbents to remove at least a portion of the haze precursors.
Haze precursors may also result from dewaxing that is not and/or cannot practically be carried out to the extent necessary to prevent haze formation. For example, leaks in solvent dewaxing filter cloths and bypassing in beds of catalysts used to dewax lubricant base stocks are inevitable and are usually difficult to detect. Leaks of much less than 1% can cause haze formation in the resulting lubricant base stock. Haze may also be caused by small inorganic particulates, such as from catalyst fines or corrosion.
There is a need to improve the filterability, haze formation or both of lubricating oil basestocks without the need for catalysts or adsorbents.